The Erythemal Index is a measure of ultraviolet radiation (UV) at ground level on the Earth. UV exists to the left of the visible spectrum and is divided into three components (UV-A, UV-B and UV-C). UV-B (290-320 wavelengths) is the most dangerous form of UV radiation that can reach ground level. Atmospheric ozone shields life at the surface from most of the harmful components of solar radiation. Chemical processes in the atmosphere can effect the level of protection provided by the ozone in the upper atmosphere. This thinning of the atmospheric ozone in the stratosphere leads to elevated levels of UV-B at ground level and increases the risks of DNA damage in living organisms. Note: Rotating Globe shows the fluctuation in the erythemal index over the course of a year (August, 2000, through July, 2001). Animator: Michael Mangos (GST). Scientist: Richard McPeters (NASA/GSFC). Platforms/Sensors/Data Sets: Earth Probe/TOMS.